In magneto-optic memories, the magnetic bit is defined by the focus of the laser used to thermomagnetically write the bit, a magnetic domain with its magnetization aligned opposite to that of the surroundings of the magnetic media. The magnetic field used to define the direction of magnetization in the bit can be applied over a large area of the media since it is not used to define the physical area of the bit. This method of writing has a major advantage over conventional magnetic recording in that it does not require flying a magnetic head in close proximity to the medium so magneto-optics provides a high density, removable media. The disadvantage of magneto-optics is that since the magnetic field is applied over a large area, the electromagnet producing the magnetic field has a high inductance and therefore cannot be switched magnetically at high data rates. The inability to switch the magnetic field applied over a large area at high data rates prevents the direct overwrite of data on any given track. It is only possible to write one type of magnetic domain, i.e. all ones, with the magnetic field applied in one direction over a large area of the media. To revise or update magneto-optic data, the track is read and stored in a semiconductor memory, the track is then erased and rewritten with the revised data on the track. After writing, the magneto-optic track is read again to verify that the revised data was written correctly.
In U.S. Pat. No. 5,164,926 which issued on Nov. 17, 1992 to H. Matsumoto, a magneto-optic recording medium with four magnetic layers is described. The recording medium provides an over-write capability by modulating only the intensity of a light beam in accordance with information to be recorded without modulating the direction and strength of a bias field. The magnetic film of four layers consists of a first layer of a perpendicularly magnetizable magnetic thin film, a second layer consisting of a perpendicularly magnetizable magnetic thin film, a switching layer of a perpendicularly magnetizable magnetic thin film, and an initializing layer consisting of a perpendicularly magnetizable magnetic thin film. The first and second layers are exchanged-coupled to each other, the second and fourth layer are exchanged-coupled to each other by way of the third layer at a temperature equal to or lower than a Curie temperature of the third layer.
In U.S. Pat. No. 5,142,513 which issued on Aug. 25, 1992 to H. Takehara et al., a magneto-optic storage medium is described having a first, second and third magnetic layer each of which is composed of an amorphous alloy consisting of a heavy rare-earth metal and a transition metal, an isolation layer composed of a non-magnetic material interposed between the first and second magnetic layers for isolating magnetic exchange interaction between the first and second magnetic layers.
In U.S. Pat. 5,184,335 which issued on Feb. 2nd, 1993 to M. H. Kryder et al., a thin film magneto-optic recording layer such as TbCo or GdTbCo which has a compensation temperature a few tens of degrees centigrade above room temperature. The need for applying an external bias magnetic field to modify previously stored information in a selected region, surrounded by a magnetic domain wall of a thin film magneto-optic media is eliminated. The magneto-optic medium is selected which produces a self-demagnetizing field within a selected region when reheated.